A system for operating a heating element for a ceramic sensor in a motor vehicle is described in U.S. Pat. No. 4,348,583. There, a constant current is fed to a heating element during a first time interval. During a second time interval, the current is pulsed so that reduced power is used for heating during the second time interval. Using this type of triggering of the heating element, a high heating power is made available during the first time interval in order to attain a desired temperature as rapidly as possible. During the second time interval, reduced power is used for heating in order to maintain the temperature.
An oxygen sensor serves to measure the oxygen content of the exhaust gas and to provide an apparatus for controlling the air/fuel ratio. Until now, the oxygen sensor was generally situated very far forward in the exhaust channel, i.e., near to the internal combustion engine, in order to guarantee that the oxygen sensor was heated rapidly by the exhaust gases of the internal combustion engine.
In order to heat the oxygen sensor even more rapidly, it is generally equipped with an electrical heating element. Moreover, it can be ensured by way of the heating element that the oxygen sensor is maintained at operating temperature even under operating conditions whereby the exhaust temperature is low and/or only a very small quantity of exhaust gases are present.
However, problems can arise if the oxygen sensor is situated near to the internal combustion engine:
First, if the internal combustion engine is operated at high power for a long time, a large quantity of very hot exhaust gases will be produced which can possibly heat the oxygen sensor to an intolerably high temperature. This can reduce the service life of the oxygen sensor. PA1 Second, it is generally difficult to find a suitable installation site for the oxygen sensor in the exhaust channel near to the internal combustion engine from which the exhaust gases from all cylinders of the internal combustion engine can be measured.
These difficulties can be circumvented by situating the oxygen sensor downstream, i.e., away from the internal combustion engine, in the exhaust channel. However, this second installation site entails a new problem. In the initial phase after starting a cold internal combustion engine, the exhaust channel upstream from the oxygen sensor will remain relatively cold. This will result in the condensation of the water contained in the exhaust gases. If the condensed water droplets are, for example, pulled loose from the wall of the exhaust passage by the exhaust gases streaming by and slung onto the oxygen sensor, the oxygen sensor will be cooled down very rapidly at the local points of impingement. This cooling can result in damage to the oxygen sensor (e.g., cracks in the ceramics). The risk of damage is particularly high if the oxygen sensor is already at a high temperature.